Contemporary internal combustion engines are based on technology developed one hundred years ago. Referring to FIG. 1, for example, pistons 10 reciprocating in cylinders 12 are attached to piston rods 14 which in turn are connected to crank 16 to convert linear motion of the piston 10 into rotary power. Each piston reciprocates in its associated cylinder, offset from the rotational axis (the Z-axis) of the drive shaft about point 17 by the length of the piston rod 14 plus the length of the crankshaft 16. The piston rod 14 moves in a linear path at the end connected to the piston 10 at point 13, and moves about the rotational (Z) axis in a circular path 19 in the X-Y plane at the crankshaft end of the piston 10 at point 15.
Unfortunately, this compound motion of the piston 10 and piston rod 14 causes vibration which is difficult to balance dynamically or statically. Multi-cylinder engines, for example, employ heavy, static counterweights in the crankshaft in an effort to smooth and balance this compound motion. This vibration problem only exacerbates as the piston moves further away from the rotational (Z) axis.
Typically, an air-fuel mixture is drawn into a sealed combustion chamber 18 where it is compressed by the motion of the piston as it is driven by the rotation of the crankshaft. This compressed air-fuel mixture is ignited, and the piston is powered for a single stroke and then carried through additional strokes in the cycle, e.g., three more strokes for a four stroke engine, by the action of a flywheel and power from the other pistons. Each piston provides only one power stroke for every two revolutions of the drive shaft. A properly timed four cylinder engine, therefore, provides only two power pulses per revolution of the drive shaft. Thus, even at optimum performance, considerable power is lost converting from input linear motion to output rotary motion.
Moreover, the physical space occupied by the piston rod and the crankshaft prevent the possible use of a second sealed combustion chamber in a given cylinder. Therefore, power can only feasibly be applied to one end of each piston in piston/piston rod/crankshaft engine configurations. Various incremental refinements in these engines have evolved over time, but the underlying mechanics of the piston/piston-rod/crankshaft mechanism remains largely unchanged and largely inefficient. Incremental improvements in efficiency of this tradition internal combustion engine are limited to balancing the reciprocating pistons and reducing the vibration caused by the non-symmetric motion of the piston/piston-rod/crankshaft assembly.
A different technique for eliminating the gyrating motion of the piston rod in the X-Y plane at the crankshaft end is based on hypocycloidal motion. Referring to FIG. 2, a hypocycloid is defined as the curve traced by a point P on the circumference of a rotating circle 20 of a radius r as it rolls without slipping constrained by a fixed larger circle 22 of radius f. If the ratio of f to r (f:r) is an integer n, the resulting curve will contain n cusps. In the illustrated example, the ratio f:r is 8:1, so there are 8 cusps in the hypocycloid.
Point P is the point where the two circles 20 and 22 meet at the start, and point P' is the first cusp, i.e., the point where the two circles meet after one revolution of the smaller rolling circle 20. Thus, the curve P-P' formed as rolling circle 20 of radius r makes its first revolution while constrained by circle 22 of radius f. The remainder of the hypocycloid is generated as rolling circle 20 of radius r completes seven more revolutions inside circle 22 of radius f.
Purely linear motion may be obtained with a hypocycloidal device using a two cusp hypocycloid with the f:r radii ratio corresponding to a ratio of 2:1. As shown in FIG. 3, the fixed point P on the circumference of the rolling circle 20 traces a straight line (P-P') which passes directly through the center of the fixed circle 22 when using a two cusp hypocycloid (f:r=2:1).
A number of attempts to achieve linear-to-rotary motion conversion using a two-cusp hypocycloid literally interpret the mathematical definition of a circle rolling inside another circle without slipping. Typically, a fixed ring gear is employed as a constraint and a guide for a planetary gear rolling around inside the ring gear. The planetary gear is in turn connected to a crankshaft to derive rotary motion from reciprocating motion of a piston. U.S. Pat. Nos. 1,056,746; 1,579,083; 3,175,544; 3,329,134; 3,994,136; 3,563,223; 3,744,324; 3,791,227; 4,970,995; and 5,233,949 are possible examples of such devices.
However, such designs are impractical. For example, U.S. Pat. No. 4,970,995 shows crankshafts with idler cranks that must move in two directions when rotated which results in both X and Y displacement of the crankshaft defeating the benefits of the hypocycloidal motion. Another drawback of such gear designs is the reliance on the gear teeth to absorb the explosive forces applied to them by a combustion engine. As a result, the gear teeth are prone to failure either by shearing off completely or slipping.
U.S. Pat. No. 3,175,544 incorporates a pair of pistons whose linear paths intersect at the axis of rotation. These pistons are straddled by two links moving in hypocycloidal fashion, which in turn are connected to cranks on the drive shaft. Again, this design is impractical and inefficient. Since each piston must provide clearance for the other at the intersection, the distance traveled by a piston is very long in relation to the length of the power stroke (approximately a 6:1 ratio). Given the intersecting travel of the pistons, it is not possible to maintain a fully sealed combustion chamber for either end of either piston. Additionally, exhaust gases of one piston would mingle with the charging gases of the mating cylinder, further reducing efficiency. Moreover, maintaining alignment of each piston with its respective cylinder is very difficult as each passes through the intersecting open space under power.
I discovered that contrary to the conventional thinking regarding internal combustion engine designs, neither gearing nor a crankshaft is necessary to constrain drive elements in a hypocycloidal path. In contrast, the present invention uses hypocycloidal mechanics to provide a "virtual crankshaft engine." Namely, the engine delivers rotary power using pistons that reciprocate symmetrically about and directly through a center drive axis. Such reciprocating pistons would interfere with the crankshaft in conventional engine designs. That rotary power is transferred to a drive shaft aligned with that center drive axis using a hypocycloidal linkage connected to an end piston. By eliminating the two axis (X-Y) displacement of the piston rods 14 at the crankshaft end (point 15), a rigid, double-ended piston can be utilized to double engine efficiency relative to crankshaft-based single cylinder engines. All components of the piston/piston-pin assembly move along a single axis in a linear path.
The virtual crankshaft engine therefore efficiently converts linear motion of reciprocating pistons into rotary motion to rotate an engine drive shaft by connecting adjacent pistons using linkages that follow a two cusp hypocycloid. The pistons are housed in corresponding cylinders positioned adjacent to one another along and perpendicular to the axis of rotation. These pistons reciprocate in their corresponding cylinders through the axis of rotation. Preferably, pairs of double-ended pistons are powered simultaneously on opposite sides of the axis of rotation to dynamically balance the engine.
In one preferred embodiment, the cylinders are parallel to one another, and a plurality of drive shaft segments are connected between adjacent cylinders by hypocycloidal linkages. Each linkage includes a piston link and a crank link. Each piston includes a piston pin connected to one end of the piston link. The other end of the piston link is rotatably connected at a rotary joint to one end of the crank link. The other end of the crank link is connected to one of the drive shaft segments.
In this embodiment, the piston link and the crank link are the same length, and a stroke length of the piston is four times the link length. Each cylinder includes a longitudinal slot along which the piston pin travels. Linear movement of the piston pin along the slot causes the rotary joint to move in a circle. As the rotary joint moves about the circle, the drive shaft segment rotates about the axis of rotation.
In another preferred embodiment, adjacent cylinders are perpendicular to each other and to the axis of rotation. The stroke length of the piston in this embodiment is twice a length of the linkage which connects the piston pins of adjacent pistons.
The virtual crankshaft engine design reduces engine weight, volume, vibration, friction, and component complexity (relative to traditional drive trains) which translates into both reduced manufacturing costs and improved performance and reliability. For example, symmetrical, sinusoidal motion of the reciprocating pistons about the drive axis of rotation improves balance and reduces vibration over crankshaft drive designs. Moreover, double-ended pistons deliver twice as much horsepower per cylinder compared to conventional single-ended piston designs.